This invention relates to suction air passage of internal combustion engines, referring more particularly to the side-valve type of internal combustion engines on which a suction valve and an exhaust valve are located side by side with each other on one side of the cylinder.
As the side-valve type of internal combustion engines is described in the U.S. Pat. No. 3,118,433, the engines can be illustrated as follows.
As shown in FIG. 1, a suction valve 1 and an exhaust valve 2 of the conventional internal combustion engine 4 are located side by side with each other on one side of the cylinder 3, with a silencer 5 placed outside of the exhaust valve 2 and with a carburetor 6 almost diagonally to the silencer 5 on the other side of the cylinder 3.
In connection with the side-valve type of internal combustion engine 4 thus arranged, a suction air passage 7 from the carburetor 6 to the suction valve 1 is a long one which consists of a suction air pipe 7a, and a joint 7b and an elbow 7b' at its both ends. Thus, the configuration needs many connections such as the carburetor 6--the joint 7b, the joint 7b--the suction air pipe 7a, the suction air pipe 7a--the elbow 7b', and the elbow 7b'--the cylinder 3, resulting in troublesome assembling of many parts, and in short rigidity of the carburetor connection as well as in incomplete air tightness at the foresaid connections. Being curved abruptly at the elbow 7b', the suction air passage 7 tends to have rather large air resistance, preventing gas mixture from being drawn smooth. Furthermore when an ignition coil is designed to be installed outside the flywheel, the coil must be located over the suction air passage 7 or at the outside in the axial direction of the crankshaft. If the suction pipe 7a is isolated from the cylinder for assembly, the crankshaft must be extremely long, because the ignition coil must be located over the isolated suction air pipe. And also the suction air pipe 7a must be curved or dented to avoid the interference between the ignition coil mounting bosses and the suction air pipe.